How mercury enters the ocean
ONCE DEPOSITED in water, mercury is often changed by microbes into methyl mercury, a known nerve toxin that can accumulate in the tissues of marine animals that consume the microorganisms and smaller fish.
Methyl mercury is particularly dangerous to the developing nervous systems of foetuses, young children, and animals.
Substantial quantities
In recent years, researchers have cautioned pregnant women to limit the consumption of certain types of fish due to concerns about mercury contamination.
How does mercury get deposited in ocean water in substantial quantities? It is mostly due to a phenomenon called `submarine groundwater discharge,' which has been receiving more attention in recent years because scientists have shown that the flow of groundwater into the ocean carries a substantial amount of dissolved nutrients, metals, and trace elements. Mercury pollution comes mostly from industrial emissions to the atmosphere, especially from coal burning. After getting into the air, mercury particles eventually precipitate with rain or snow onto the land or directly into the oceans.
Inland deposits of mercury are also weathered and carried to the coast in runoff from streams and rivers, where they accumulate in the sediments that build up along the shoreline.
At the same time, wherever aquifers are connected to the ocean, fresh groundwater can be discharged and salty sea water can penetrate landward into groundwater— both passing through and picking up this mercury embedded in the sediments (submarine groundwater discharge).
Researchers from the Woods Hole Oceanographic Institution (WHOI) have found this new and substantial pathway for mercury pollution flowing into coastal waters.
Marine chemists have detected much more dissolved mercury entering the ocean through groundwater than from atmospheric and river sources.
Mercury is toxic to animals and humans in large concentrations, particularly in the form known as methyl mercury, which accumulates in fish.
To date, WHOI researchers examined total mercury, not the more biologically dangerous form, though that is a logical next step.
Quantifying impact
These initial findings of mercury moving through the coastal groundwater system are significant for researchers trying to quantify the impact of mercury in the marine environment.
The lead author of the study is Sharon Bone, a former undergraduate summer student fellow and research assistant in the laboratory of WHOI marine chemist Matt Charette.
Bone is now a first-year graduate student at the University of California at Berkeley. The findings were published online by the journal Environmental Science and Technology.
"This pathway for delivering nutrients and contaminants into the ocean has long been overlooked and ignored because it was difficult to quantify," said Charette, whose lab has advanced such methods in recent years.
First of its kind
"This study is a first of its kind for quantifying the amount of mercury flowing out of the system."
Bone and colleagues started by analysing cores of coastal sediments, observations from shoreline wells, and measurements of submarine groundwater flow to determine the amount of mercury flowing out of the subterranean estuary. Then, while sampling surface waters, Bone detected concentrations of dissolved mercury in the bay that were much higher than would be expected from simple atmospheric deposition and runoff, according to a WHOI press release.
Unbelievable
Charette and fellow chemist Carl Lamborg of the WHOI Department of Marine Chemistry and Geochemistry could not believe what they were finding.
After checking and rechecking their methods and data, the research team found total mercury concentrations that were an order of magnitude (at least 10 times) higher than what should be deposited by simple outfall from the atmosphere, and substantially more mercury than could flow in from local streams.
The source had to be submarine groundwater pushing mercury out from the sediments.
OUR BUREAU
Printer friendly
page
Send this article to Friends by
E-Mail
Sci Tech
